1. Field of the Invention
The present invention relates to an electrode structure and a method of fabricating the same, and more particularly, it relates to an electrode structure employed as a gate electrode of a semiconductor device and a method of fabricating the same.
2. Description of the Prior Art
A structure formed by a first layer of polysilicon, for example, and a second layer, located on the first layer, of metal silicide is recently employed for a gate electrode forming a semiconductor device, in order to reduce electric resistance. When such a gate electrode is applied to a DRAM (dynamic random access memory), for example, the gate electrode is first formed by etching, and side walls of the gate electrode and the upper surface of a semiconductor substrate are thereafter covered with a thermal oxide film for improving hot carrier resistance. When this step is employed, however, side surfaces of the second layer made of metal silicide such as tungsten silicide are more readily oxidized as compared with side surfaces of the first layer made of polysilicon, and hence oxide films abnormally grow on the side surfaces made of metal silicide. For example, Japanese Patent Laying-Open No. 7-183513 describes a technique of preventing such abnormal growth of oxide films.
FIG. 10 is a sectional view of a semiconductor device having a conventional electrode structure described in the aforementioned gazette. Referring to FIG. 10, insulator films 101 for element isolation are formed on a silicon substrate 100 in the conventional semiconductor device. Between the insulator films 101 for element isolation, a polysilicon film 103 is formed on the silicon substrate 100 through a gate oxide film 102. A source region 110 and a drain region 111 are formed in the silicon substrate 100 on both sides of the polysilicon film 103. Rounded portions 108 and bird""s beaks 109 are formed on the side walls of the polysilicon film 103 to be in contact with the polysilicon film 103.
A natural oxide film 104 is formed on the polysilicon film 103. A tungsten silicide film 105 is formed on the natural oxide film 104. A silicon nitride film 106 is formed on the tungsten silicide film 105.
Silicon nitride films 107 are formed to be in contact with the silicon nitride film 106, the tungsten silicide film 105 and the natural oxide film 104.
This electrode structure is fabricated as follows: First, the gate oxide film 102 is deposited on the silicon substrate 100, and the polysilicon film 103, the natural oxide film 104, the tungsten silicide film 105 and the silicon nitride film 106 are deposited thereon in a layered manner. The silicon nitride film 106, the tungsten silicide film 105 and the natural oxide film 104 are patterned into the shapes shown in FIG. 10. The silicon nitride films 107 are formed to cover the silicon nitride film 106, the tungsten silicide film 105 and the natural oxide film 104, and fully etched back into the shapes shown in FIG. 10. Thereafter the polysilicon film 103 is etched into the shape shown in FIG. 10, and thereafter the side walls thereof are oxidized to form the rounded portions 108 and the bird""s beaks 109.
According to this method, the side walls of the tungsten silicide film 105, covered with the silicon nitride films 107 in the oxidation step, are inhibited from abnormal oxidation. On the other hand, the side walls of the polysilicon film 103 covered with no nitride films are thermally oxidized through the oxidation step. Consequently, the gate electrode can be improved in reliability.
However, the conventional technique has the following problem: The silicon nitride films 107 are formed on the side walls of the tungsten silicide film 105. The width of the tungsten silicide film 105 is reduced due to the silicon nitride films 107. Therefore, the tungsten silicide film 105 is reduced in sectional area and increased in electric resistance. Thus, the reliability of the electrode structure is deteriorated due to a signal delay or the like.
Accordingly, the present invention has been proposed in order to solve the aforementioned problem, and an object thereof is to provide a reliable electrode structure.
The electrode structure according to the present invention includes a first conductive layer having a first side wall and containing at least either polycrystalline silicon or amorphous silicon, a second conductive layer, formed on the first conductive layer, having a second side wall and containing a metal and silicon, and a side wall oxide film formed to be in contact with the first side wall and the second side wall. The first conductive layer and the second conductive layer contain nitrogen in the vicinity of the first and second side walls. The nitrogen concentration in the second side wall is larger than the nitrogen concentration in the first side wall.
In the electrode structure having the aforementioned structure, the side wall oxide film is formed to be in contact with the first side wall and the second side wall, whereby the first and second side walls can be prevented from crystal defects. The nitrogen concentration in the second side wall is larger than the nitrogen concentration in the first side wall, whereby larger quantities of nitrogen are added to the second side wall containing the metal and silicon. Consequently, the growth rate of the side wall oxide film can be retarded on the second side wall, so that the side wall oxide film can be prevented from abnormal growth on the second side wall. Further, the side wall oxide film is formed on the second side wall so that no silicon nitride films are formed to be directly in contact with the second side wall, whereby a sufficient width can be ensured for the second conductive layer. Consequently, a reliable electrode structure having small conductive resistance can be provided.
Preferably, the nitrogen concentration in the second conductive layer is increased toward the second side wall.
Preferably, the nitrogen concentration in the first side wall is substantially identical to the nitrogen concentration in a central portion of the first conductive layer.
Preferably, the first conductive layer is formed on a semiconductor substrate.
Preferably, the electrode structure further includes a gate oxide film formed between the first conductive layer and the semiconductor substrate.
Preferably, the electrode structure further includes a surface oxide film formed on the semiconductor substrate continuing to the gate oxide film and the side wall oxide film.
Preferably, the first conductive layer has a portion reduced in width toward the semiconductor substrate. The electrode structure further comprises impurity regions formed on the semiconductor substrate on both sides of the first conductive layer.
Preferably, the electrode structure further includes a silicon nitride film formed to cover the side wall oxide film.
A method of fabricating an electrode structure according to the present invention includes steps of successively stacking a first layer containing at least either polycrystalline silicon or amorphous silicon and a second layer containing a metal and silicon, forming a second conductive layer by etching the second layer, doping the side walls of the second conductive layer with nitrogen by exposing the second conductive layer to an atmosphere containing nitrogen, forming a first conductive layer by etching the first layer through a mask of the second conductive layer having the side wall doped with nitrogen, and forming a side wall oxide film by oxidizing the side walls of the first and second conductive layers.
In the method of fabricating an electrode structure having the aforementioned structure, the side wall of the second conductive layer is doped with nitrogen for thereafter forming the first conductive layer by etching the first layer through the mask of the second conductive layer doped with nitrogen. Therefore, the side wall of the second conductive layer is doped with large quantities of nitrogen, while the side wall of the first conductive layer is hardly doped with nitrogen. Consequently, the side wall of the second conductive layer can be inhibited from abnormal oxidation in the step of forming the side wall oxide film by oxidizing the side walls of the first and second conductive layers. Further, the side wall oxide film is formed on the side wall of the second conductive layer, whereby the width of the second conductive layer can be increased as compared with a case of forming a silicon nitride film to be in contact with the side wall of the second conductive layer. Consequently, the second conductive layer can be increased in sectional area and reduced in conductive resistance, for providing a reliable electrode structure.
Preferably, the step of successively stacking the first layer and the second layer includes a step of successively stacking the first layer and the second layer on a semiconductor substrate.
Preferably, the method of fabricating an electrode structure further includes a step of implanting an impurity into the semiconductor substrate through masks of the first and second conductive layers after forming the side wall oxide film.
Preferably, the method of fabricating an electrode structure further includes steps of stacking a silicon oxide film and a silicon nitride film on the second layer and etching the silicon oxide film and the silicon nitride film. The step of forming the second conductive layer includes a step of etching the second layer through a mask of the etched silicon nitride film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.